In order to lighten a turbomachine, some of the components thereof which are conventionally produced from metal may be made from a composite material. Structural elements such as the supports and casings may be made from a composite material. In particular, an external casing of a compressor, notably a low-pressure compressor, may be produced from a composite material.
Such a casing generally has a cylindrical wall supporting annular rows of blades, and annular fixing flanges extending radially. The flanges allow the compressor casing to be mounted on an intermediate fan casing and allow the fixing of a dividing annuler lip. The internal surface of the casing guides the annular flow along the compressor and for that reason needs to have a smooth surface, without steps. This constraint means that sharp-edged corners need to be produced at the fixing flanges.
A compressor composite external casing may be produced from a preform densified by a matrix. The preform may be formed by a stack of fibrous sheets stacked on top of one another, the sheets extending both over the cylindrical portion and onto the fixing flanges. Once shaped, the preform is placed in an injection mould that has edge corners to reproduce the desired shape of the casing. At the edge corner of the mould the stacked preform forms a fillet radius and leaves, in this region, a space that is unoccupied by fibers, or, at the very least, lacking in fibers. At the time of injection, the space is filled with resin without being reinforced by fibers. As a result, the composite casing exhibits a weakness at this point.
Document EP 1 900 502 A1 discloses a turbomachine composite annular casing. The casing has a tubular body and annular flanges at the ends of the body. The flanges form annular sharp-edged corners on the inside of the body. The composite casing comprises several fibrous reinforcements one of which is unraveled in the region of the edge corners so that the fibers there may be spread out in the joins between the flanges and the annular body. This unraveling is performed by means of a step of raising the temperature and allows the fibers to be spread out effectively in the edge corners. However, the unraveling step ties up the mould for a period of time that reduces the casing production rate. This teaching also entails the use of plies that are capable of being unraveled.